text: There are two types of conductivity detectors, direct and membrane.
  Direct conductivity provides an all-encompassing approach of measuring CO.  This detection method uses no carrier gas, is good at the parts per billion (ppb) ranges, but has a very limited analytical range.
  Membrane conductivity relies upon the filtering of the CO prior to measuring it with a conductivity cell. Both methods analyze sample conductivity before and after oxidization, attributing this differential measurement to the TOC of the sample.  During the sample oxidization phase, CO (directly related to the TOC in the sample) and other gases are formed. The dissolved CO forms a weak acid, thereby changing the conductivity of the original sample proportionately to the TOC in the sample. Conductivity analyses assume that only CO is present within the solution. As long as this holds true, then the TOC calculation by this differential measurement is valid.  However, depending on the chemical species present in the sample and their individual products of oxidation, they may present either a positive or a negative interference to the actual TOC value, resulting in analytical error.  Some of the interfering chemical species include Cl, HCO, SO, SO, ClO, and H.   Small changes in pH and temperature fluctuations also contribute to inaccuracy.
  Membrane conductivity analyzers have improved upon the direct conductivity approach by incorporating the use of hydrophobic gas permeation membranes to allow a more “selective” passage of the dissolved CO gas and nothing else. This provides a more precise and accurate measurement of the organics that were converted to CO.